Researchers from the University of Massachusetts at Amherst asked more than 100 healthy adults to rate their emotional responses to a series of images, some depicting unsettling scenes. Twelve hours later, they rated the images again. The difference: Half of the subjects slept during the break; the other half did not.

"Not only did sleep protect the memory, but it also protected the emotional reaction," said Rebecca Spencer, a neuroscientist at UMass Amherst and co-author of the study that was published in the Journal of Neuroscience.

Study subjects who stayed awake for 12 hours had a weaker emotional response to the unsettling images the second time around, suggesting sleep serves to preserve and even amplify negative emotions. Their memories were also weaker than those of their well-rested counterparts, as they struggled to remember whether they had seen the images before.

"It's true that 'sleeping on it' is usually a good thing to do," said Spencer, citing evidence that sleep boosts memory and other cognitive functions. "It's just when something truly traumatic or out of the ordinary happens that you might want to stay awake."

Spencer said people often find it difficult to sleep after a traumatic event.

"This study suggests the biological response we have after trauma might actually be healthy," she said. "Perhaps letting people go through a period of insomnia before feeding them sleeping meds is actually beneficial."

While the findings may have implications for post traumatic stress disorder, Spencer emphasized that daily emotional ups and downs are not grounds for sleep deprivation.

"Just because we have a bad day doesn't mean we should stay awake," she said. "We need to maintain some memories and emotional context to know what to avoid. We do learn something from them."

Although sleep gives the body some much-needed rest, the brain stays active. Spencer used polysomnography to monitor brain activity in some sleeping subjects.

"REM sleep in particular was associated with a change in how emotional you found something," she said. "We think there are parts of the brain being activated during sleep that allow us to process those emotions more than during day."

Next, Spencer plans to study the link between sleep and memory in the context of aging. With age, the amount of time spent sleeping drops dramatically.

"We want to know if those changes actually underlie some of the cognitive and behavioral changes that occur with age," she said.